Executive function refers to the higher order cognitive processes that are responsible for planning, executing, and evaluating problems. The development of executive function is widely studied as it has broad implications for tracking typical and atypical development, including ADHD. However, due to limitations in current methodology, most developmental accounts of executive function ignore the abilities prior to 3 years of age. The current proposal puts forth a unified account of the development of executive function through the framework of a computational model of search behavior. Using variants of the A-not-B search task, the development of executive function may now be tracked from infancy, through toddlerhood and into the early childhood years. In an A- not-B task, the child must search correctly at a new location (B) after previously searching a number of times at an old location (A); failure to do so is termed the A-not-B error. The computational model that guides this research program consists of a function that captures the effects of habit (i.e., previously executed responses) and a function that captures the ability to reflect on representations (e.g., reminding one's self where the object is hidden). Interestingly, this model generates a number of testable hypotheses that all stem from the unique perspective that task experience simultaneously increases the likelihood of searching incorrectly due to habit and decreases the likelihood of searching incorrectly by offering additional opportunities for reflection. The proposed research will expand on this hypothesis by testing children across the preschool age range (i.e., 2 to 4 years), and varying the likelihood of reflection via labeling cues. By furthering our understanding of the joint develolpment of habit, memory, and language use, we can begin to track the development of executive function over the first few years of life. This can serve as a basis for the early typical development of executive function, and may serve as a means to detect atypical trajectories that have been implicated in autism, attention deficit hyperactivity disorder, and maltreatment related posttraumatic stress disorder. This research will study directly the combined develolpment of habit, memory, and language use, which will permit the tracking of the development of higher order cognition over the first few years of life. This can serve as a basis for the early typical development of executive function, and may serve as a means to detect atypical trajectories that have been implicated in autism, attention deficit hyperactivity disorder, and maltreatment related posttraumatic stress disorder. [unreadable] [unreadable] [unreadable] [unreadable]